Ballasted storm water retention system

ABSTRACT

A roof system designed to retain water during storms and release it slowly over a period of time includes a polymeric water impervious membrane covered with a drainage board in turn covered with a water retention mat. The water retention mat and drainage board are held in position by ballast, preferably gravel. The mat is designed to retain at least about 25% of a one-inch per hour rainfall and release it gradually over 2 to 3 hours, thereby reducing rain runoff during storms and reducing the load on a storm sewer system

RELATED APPLICATION

This application is related to and claims the benefit of U.S. Provisional Patent Application Ser. No. 61/023,257, filed Jan. 24, 2008, the disclosure of which is incorporated herein by reference.

BACKGROUND OF THE INVENTION

Storm runoff presents a significant burden to sewage systems. Excess water can exceed the capacity of the storm sewer system. Drainage systems that include holding ponds can be used to temporarily retain water and slowly feed this to sewer systems. Such drainage systems can be relatively expensive, particularly as their volume increases.

One way to reduce the storm runoff is to temporarily retain water on a roof using a garden roof. The garden roof incorporates a growth medium layer, which supports plant growth and also acts to retain water. Thus, during a storm, the growth medium absorbs water and gradually allows some of the water to run off the roof. This slows the storm runoff, allowing the storm sewer system to handle the water over a longer period of time.

Unfortunately, garden roofs are in many applications cost prohibitive. Further, the maintenance costs of a garden roof must also be considered.

SUMMARY OF THE INVENTION

The present invention is premised on the realization that storm water runoff from a flat roof can be reduced utilizing a ballasted storm water retention roof system. The storm water retention roof system incorporates a water impervious roofing membrane as the water barrier for the roof. This membrane may be covered with a drainage board in turn covered with a water retention mat. The drainage board and mat are held in position with a ballast layer such as gravel.

The water retention mat holds a significant amount of water from a storm, and releases this water over an extended period of time. The water retention mat can be formed from various materials such as nonwoven polymeric materials including recycled plastic. The particular material selected, as well as the thickness of this material, determine the amount of water retained, as well as the retention period. Further, the drainage board prevents ponding of water.

The objects and advantages of the present invention will be further appreciated in light of the following detailed description and drawings in which:

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 is a cross sectional view of a roof system according to the present invention;

FIG. 2 is a perspective view partially broken away of the water drainage board used in the present invention.

DETAILED DESCRIPTION

As shown in FIG. 1, the present invention is a roof structure 10 that incorporates a roof deck 12 covered with a water barrier layer 14. The roof structure 10 is a flat roof, meaning the slope of the roof is less than about 1 inch per foot. Generally, the slope is about 0.25 inch/foot. As shown, the water barrier layer 14 is a polymeric membrane, although any water barrier layer that is suitable for a flat roof surface can be used in the present invention. Suitable polymeric membranes include EPDM, TPO and polyvinylchloride. These are commonly used on flat roof surfaces. The water barrier layer 14 can be installed in any typical manner. For example, the membrane can be simply loose-laid, held with mechanical fasteners, or fastened with an adhesive.

Between the roof deck 12 and the membrane 14 can be various layers, as is well known. Typically, one may employ an insulation layer 15 between the roof deck and the membrane.

Covering the membrane 14 is an optional, protective layer 17, which can be any layer that is typically employed as a protective layer for a roof surface. These protective layers are generally semi-rigid mats, and are typically the type employed for a garden roof system. Typical protective layers can be formed from tufted nonwoven polypropylene fibers. One such material is sold by Carlisle Coating & Waterproofing, CCW 200 V, which is a thick, nonwoven, cushioning fabric formed from polypropylene. Any stable material which protects the membrane from the overlaid drainage board can be used as necessary.

The protective layer 17 is, in turn, covered with a drainage board or member 16. A drainage board simply provides water flow channels above the membrane 14, which prevents ponding. This layer may be excluded. Drainage board 16, in turn, is covered with a water retention mat 18, which is, in turn, covered with ballast 20.

The drainage board 16, as shown, includes an upper mesh layer 26 and a spacer layer 24. The spacer layer 24 is a solid plastic member that has a series of truncated conical spacers 25. A series of holes 22 in the upper surface 23 of the spacer layer 24 allows water flow from above drainage board 16 to channels 27. The channels 27 within the layer between spacers 25 permit water flow. This is shown in more detail in FIG. 2. The channels 27 in board 16 can provide a flow rate of 140-145 gpm/ft2 (ASTM D 4491).

The spacing and the height of the channels 27 are designed to provide sufficient area for the desired water flow. Typically, the spacers will provide about a ¾-inch space between the upper and lower surfaces. The upper layer 26, which is fixed to spacer layer 24, supports the water retention mat 18 and allows water to pass through to channels 27 from the retention mat 18. The upper layer 26 can be any water pervious layer that is strong enough to support the retention mat and gravel. This can be a nonwoven mat, a woven mesh, or a perforated film.

One preferred drainage board is manufactured by Carlisle Coating & Waterproofing, sold under the name CCW Miradrain GR9200. This product is a high-performance retention composite with a three-dimensional, high-impact polystyrene core, which forms the layer 14 and the spacers 25. The upper layer includes holes 22, which permit water flow. Generally, about 150⅛-inch holes every square foot is acceptable. Another such product is CCW Miradrain HC Drainage Board, which also has a similar structure.

These are one-piece drainage boards. The water flow channels can also be provided using a variety of different structures, which combine to form water drainage channels and support the water retention mat 18 and gravel 20.

The water retention mat 18 is a porous mat, which is designed to retain water during periods of heavy water flow. Water retention mats are also referred to as water retention geotextiles. Retention mats are typically used in garden roofs to provide a water reservoir for plants. These mats act as sponges, holding large quantities of water for use by the plants. The water retention mats can be formed from fibers and can be woven or nonwoven. The retention mat can also be formed from other porous materials, such as open celled foams. The thickness of the water retention mat is selected to retain the desired amount of water.

There are a number of products on the market that will function as water retention mats in the present invention such as products sold by Huesker, Inc., HGTA-8-10-TTSPET, HGTA-10-10-10-PET, CCW 200V, CCW 300V Protection Mat, and Carlisle SynTec Moisture Retention Mat, as well as almost any geotextile.

The moisture retention mat is designed to hold ten times its weight in water and serve to slowly release the precipitation through capillary action. It is formed from recycled fibers that are held together in a tufted nonwoven web. It has a dry weight of 0.18 pounds per square foot, a water-saturated weight of 1.75 pounds per square foot, and holds approximately 0.2 gallons of water per square foot.

Again, depending upon the desired water retention, one or more layers of any particular water retention mat can be used to achieve the desired water retention. The system should be designed to retain at least 25% of the water from a one-inch per hour rainfall, and release this over a period of three hours. However, the system can be designed to retain less or more water, depending upon the desired results.

The final course of the roof system is the ballast 20, which holds the water drainage system, including the water retention mat 18 and the drainage board 16 and any protective layer 17 onto the membrane surface. The ballast 20 can also be used to hold the membrane 14 in position if the membrane 14 is not otherwise fastened to the roof deck surface.

The ballast 20 is typically washed creek stone or gravel. Other ballast would include paver stones, and the like. With respect to the present invention, the term “ballast” specifically excludes dirt or other growth medium used in a green roof surface. Typically, gravel is employed for the ballast. The mass of the ballast should be 10 to 24 lbs/ft², generally about 17 lbs/ft².

The roof structure of the present invention is constructed using standard roof installation procedures to apply the water impervious layer, such as the membrane and any insulation over the roof deck. More specifically, the protective layer 17 is positioned over membrane 14. The drainage board 16 is positioned over the protective fabric with the spacers 25 facing upwardly, as shown. The drainage boards 16 are not positioned within 12 inches from curbs, parapets, drains, and other wall penetrations. The moisture retention mats 18 are positioned over the drainage board 16 with edges of adjacent moisture retention mats overlapped at least two inches. If multiple layers of moisture retention mats are used, the seams are staggered by, for example, twelve inches to prevent gravel migration, and to increase retention capacity.

Rounded water-worn gravel is used as ballast and placed directly over the moisture retention mat with a minimum of ten pounds per square foot, depending on the wind load requirements. Crushed stone is not advised for use as a ballast as the small particles can clog the fabric.

Once installed, this roof system will help to control rainwater runoff. In a heavy downpour, the rain will soak the moisture retention mat 18. Once the mat is saturated, any additional rainwater will immediately flow into channels 27 or over the gravel surface. Once the rain has stopped, the water remaining in mat 16 will slowly drip into channels 27 or simply evaporate.

The roof system of the present invention provides several advantages. In particular, it acts to retain water from a downpour and release it slowly over an extended period of time. Further, it accomplishes this without the significant added expense of a green roof. Further, the structure of the present invention prevents pooling of water, which would occur is one were simply to use a water retention mat by itself, over a membrane roof.

This has been a description of the present invention along with the preferred method of practicing the present invention. However, the invention itself should only be defined by the appended claims, WHEREIN WE CLAIM: 

1. A roof system comprising in the following order: a roof deck; a waterproof layer covering said roof deck; a drainage member providing flow channels covering said waterproof layer; a moisture retention mat supported by said drainage member; and ballast covering said moisture retention mat.
 2. The roof system claimed in claim 1 wherein said drainage member comprises rigid spacers fixed to a water permeable layer.
 3. The roof system claimed in claim 2 wherein said moisture retention mat comprises a nonwoven polymeric mat.
 4. The roof system claimed in claim 3 wherein said nonwoven polymeric mat retains at least about 0.2 gallons of water per square foot.
 5. The roof system claimed in claim 2 wherein said ballast comprises gravel.
 6. The roof system claimed in claim 1 comprising an insulation layer between said roof deck and said waterproof layer.
 7. The roof system claimed in claim 1 further comprising a protective mat between said drainage member and said waterproof layer.
 8. The roof system claimed in claim 3 wherein said waterproof layer comprises a polymeric membrane.
 9. A roof system wherein the outermost three layers consist of ballast covering a water retention matting covering a water drainage member.
 10. A roof system comprising in the following order: a roof deck; a waterproof layer covering said roof deck; a drainage member providing water flow paths; a moisture retention mat supported by said drainage member; and ballast covering said mat. 